1. Field of the Invention
The present invention relates to an organic electro-luminescence (hereinafter simply referred to as “EL”) display device, and, in particular, to a top-emission type organic EL display device in which generation of dark spots by moisture is inhibited and which is highly reliable.
2. Description of the Related Art
Organic EL display devices include a bottom-emission type in which light emitted from an organic EL layer is emitted in a direction toward a glass substrate over which the organic EL layer or the like is formed and a top-emission type in which the light emitted from the organic EL layer is emitted in a direction opposite to that toward the glass substrate over which the organic EL layer or the like is formed. The top-emission type organic EL display device has an advantage in that the brightness of the display can be improved because a larger area can be secured for the organic EL layer.
In the organic EL display device, an organic EL layer is sandwiched between a pixel electrode (lower electrode) and an upper electrode, a constant voltage is applied to the upper electrode, a data signal voltage is applied to the lower electrode, and an image is formed by controlling light emission in the organic EL layer. The supply of the data signal voltage to the lower electrode is achieved through a thin film transistor (TFT). In the top-emission type organic EL display device, because the organic EL layer can be formed also over the TFT or the like, the light emission area can be increased.
The light emission characteristic of an organic EL material used in the organic EL display device is degraded when moisture is present, and, after an operation for a long period of time, the location which is degraded by the moisture stops emitting light. This appears as a dark spot in the display region. The dark spot grows as time elapses, and becomes a defect in an image.
In order to prevent the generation or the growth of the dark spot, it is necessary to remove the moisture in the organic EL display device. For this purpose, an element substrate over which the organic EL layer is formed is sealed with a sealing substrate, and intrusion of moisture from the outside into the organic EL display device is prevented. On the other hand, in order to remove moisture which entered the organic EL display device, a desiccant is provided in the organic EL display device.
JP2000-195661A discloses a structure in which a mixture of a desiccant and an organic compound is applied inside the sealing substrate. JP2002-33187A discloses a structure in which an organic metal compound is applied inside the sealing substrate as a desiccant. JP2006-4721A discloses a structure in which a material which is identical to the material of the organic EL layer is used as a desiccant or a moisture capturing agent.